Improvements to the checkoverflow option

[jacob navia]

(1) I have modified the code generator to generate a jump to an
error label if there is an overflow, instead of generating
code to jump over error code if there is NO overflow.

This reduces the overhead of overflow checking from 4% to almost
zero (not measurable in my setup).

(2) I have added a #pragma overflowcheck (On/Off)
This pragma turns on/off overflow checking. Like other pragmas
of lcc-win, you can also use:
#pragma overflowcheck(push,on) // Turns on checking and pushes
// the current state
#pragma overflowcheck(pop) // Pops the previous state

This will be available this week.